Conventionally, a rotating electric machine is used as a power-assist source for a power steering device. For example, the rotating electric machine of a power steering device described in Japanese patent No. 4,679,783, a front end cap and a rear end cap respectively covering front and rear ends of a pipe-shaped motor case are fastened together by two through bolts.
The rotating electric machine is configured to have a structure including a flange of the front end cap and a flange of the rear end cap fastened together by two through bolts. Therefore, the axial force for fastening each of the two through bolts is greater than the axial force of a similar structure having three or more bolts. As a result, the through bolts must have a greater outer diameter. Further, the through bolts are positioned radially outside of the motor case. Therefore, the total overall outer diameter of the rotating electric machine, including the through bolts, may increase resulting in a greater overall volume and/or size of the rotating electric machine.
Further, the two through bolts of the rotating electric machine are positioned unevenly around the circumference of the front and rear end caps. Therefore, the axial force caused by the fastening of the through bolt is unevenly applied around the circumferences of the front and the end caps. In such a manner, the front and rear end caps may be coaxially misaligned causing misalignment of the shaft with respect to the bearings within the front and rear end caps and results in an increase of an airgap between the rotor and the stator. As a result, the output of the rotating electric machine may decrease.
Moreover, since the rotating electric machine is used as the driving source of a power steering device in a vehicle, the rotating electric machine may be shaken or vibrated as the vehicle travels, for example. Stress from such vibration may be concentrated at the two fastening positions of the front and rear end caps where the two through bolts fasten the front and rear end caps. Therefore, such fastening positions on the flanges may become deformed and/or broken. To prevent deformation and/or breakage, the flanges may be reinforced resulting in flanges of greater size. However, reinforcing the flanges may result in larger front and rear end caps causing the rotating electric machine to have a greater total volume and/or size.